Method of preparing metallic nanoparticles and materials thus obtained

ABSTRACT

The invention relates to a method of preparing metallic nanoparticles and to the materials thus obtained. More specifically, the invention relates to a method of preparing metallic nanoparticles consisting in: selecting a precursor from the salts, hydroxides and oxides of metallic elements that can be reduced at temperatures below the clay silicate network destruction temperature; and depositing said precursor on a support selected from pseudolaminar phyllosilicate clays. According to the invention the method comprises: (i) a deposition step in which the precursor is deposited on the support: (ii) when the precursor is selected from among salts and hydroxides, a thermal decomposition step in a controlled atmosphere, in which the precursor is subjected to a decomposition process and is transformed into an oxide of the metallic element: and (iii) a reduction step in which the oxide of the metallic element is subjected to a reduction process in a controlled atmosphere. The aforementioned method is performed at temperatures below the clay silicate network destruction temperature.

RELATED APPLICATIONS

The present application is a continuation of Co-pending PCT ApplicationNo. PCT/ES2004/000441, filed on Oct. 15, 2004, which in turn, claimspriority from Spanish Application Serial No. 200302396, filed on Oct.15, 2003. Applicants claim the benefits of 35 USC §120 as to the PCTapplication, and priority under 35 USC §119 as to the said SpanishApplication, and the entire disclosures of both applications areincorporated herein in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention refers generally to the nanoparticles field,particularly to the field of metallic nanoparticles, and more especiallyto the area of nanoparticles homogeneously dispersed over supports.

PRIOR ART

Nanomaterials or nanostructured materials are heterogeneous systemsconstituted by particles which size ranges between 1 and 100 mm (10⁻⁹ to10⁻⁷ m). These systems have physical properties that can be verydifferent from those found in equivalent systems which size is that of amicrometric grain. Amongst the most notable physical properties thatappear at the nanometric scale are the quantization phenomena (charge,electronic levels . . . ); the confining phenomena (electronic,dielectric [Flytzanis, C., Hache, F., Kelin, M. C., Ricard, D., andRoussignol, Ph., “Nonlinear Optics in Composite Materials”, Prog.Optics, 29, 322 (1991)]; the presence of monodomains (crystalline,ferromagnetic [B. D. Cullity. “Introduction to Magnetic Materials”,Addison-Wesley, California, 1972, 117-119 and 309-311],ferroelectrical); giant magnetorresistance effect [J. I. Gittleman, Y.Goldstein, and S. Bozowski, Phys. Rev. B 5 3609 (1972)]; Hall-Petcheffect or suppression of dislocation pileups, [H. Gleiter, Progress inMater. Sci., 33, 223 (1989); V. G. Gryzanov and L. I. Trusov, ProgressMater. Sci., 37 289 (1993)], etc.

Currently there is a great effort in global R+D focused both on theproduction and the characterization of these nanoparticled materialswith the further objective to manufacture new products and devices.

Particularly, the metallic nanoparticles are one of the most studiednanomaterials since they display physical properties that are unique toinsulators and conductors or a mixture of both [H. Gleiter, Progress inMater. Sci., 33, 223 (1989), V. G. Gryaznov and L. I Trusov, ProgressMater. Sci., 37 289 (1993)]. These materials are currently being used incolloidal and catalysis chemical processes. On the other hand theexpectation is that in the near future metallic nanomaterials may beused to manufacture “opto” and/or electronic devices.

Currently, the synthesis of nanoparticles is achieved by severalmethods, such as: mechanical activation [Eric Gaffet, Fréderic Bernad,Jean-Claude Niepce, Fréderic Charlot, Chirstophe Gras, Gérard Le Caér,Jean-Louis Guichard, Pierre Delcroix, Alain Mocellin and OlivierTillement. “Some Recent Developments in Mechanical Activation andMechanosynthesis”, Journal of Material Chemistry, 9, 305-314 (1998)];wet synthesis methods (hydrothermal and thermal decomposition of theprecursor material), sol-gel [D. G. Morris, “Mechanical Behavior ofNanostructured Materials”, Vol. 2 of Materials Science Foundations,Trans Tech Publications Ltd, (1998)]; synthesis during the gaseousphase, electrochemical methods [Ebrahimi, F. Bourne, G. R. Kelly, M. S,and Matthews, T. E., Nanostruct. Mater., 1999, 11 343]; chemicalepitaxial growth (Veprek, S. J., Vac. Sci. Technol. A, 1999 17 2401];(CVD (Chemical Vapor Deposition), or by molecular beams [PhilipMoriarty, Nanostructured Materials, Reports on Progress in Physics, 64],297-381 (2001)] (MBE, Molecular Beam Epitaxy), ion sputtering procedures[J. Musil, I. Leipner, M. Kolega, Surf. Coat. Tech, 115, 32-37, (1999)],etc.

Depending on the type of material obtained these techniques are groupedas epitaxial techniques (MBE, CVD and ablation) and massive techniques(all others). However, one of the main problems encountered whenpreparing these materials is their tendency to cluster, causing thedisappearance of the properties inherent to the nanometric dimensions.In the only instances in which a good control of the microstructure hasbeen achieved (such as in MBE), the quantity of material prepared isvery small, which considerably increases the possible manufacturingcosts and prevents a viable industrial exploitation. This is the reasonthat has prompted several groups to dedicate considerable efforts toobtain perfectly dispersed nanocrystals in oxidic matrixes [K. Niihara,“New Design Concept of Structural Ceramics-Ceramic Nanocomposites”, J.Ceram. Soc. Jpn. 99 (1991) 974; S. T. Oh, M. Sando and K, Niihara,“Mechanical and magnetic properties of Ni—Co dispersed Al₂0₃nanocomposites”, J. Mater. Sci. 36 (2001) 1817; T. Sekino and K.Niihara, “Microstructural characteristics and mechanical properties forAl₂0₃/metal nanocomposites”, Nanostructural Materials, Vol. 6 (1995)663; T. Sekino, T. Nakajima, S. Ueda and K. Niihara, “Reduction andSintering of a Nickel-Dispersed-Alumina Composite and its Properties”,J. Am. Ceram. Soc., 80, 5 (1997) 1139; M. Nawa, T. Sekino and K.Niihara, “Fabrication and Mechanical Properties of Al₂0₃/MoNancomposites”, J. Mater. Sci., 29 (1994) 3183; S. T. Oh, T. Sekino andK. Niihara “Fabrication and Mechanical Properties of 5% vol CopperDispersed Alumina Nanocomposite”, J. Eur. Ceram. Soc., 18 (1998) 31; R.Z. Chen and W. H. TUan, “Pressureless Sintering of Al₂0₃/NiNanocomposites”, J. Eur. Ceram. Soc., 19 (1999) 463; K. Niihara, T.Sekino, Y. H. Choa, T. Kusunose, Y. Hayashi, K. Akamatsu, N. Bamba, T.Hirano and S. Ueda “Nanocomposite Structural Ceramics with AdvancedProperties”, Proc. 4th Japan International SAMPE (1995); K. Niihara, T.Sekino, Y. H. Choa, T. Kusunose, Y. Hayashi, K. Akamatsu, N. Bamba, T.Hirano and S. Ueda, “Nanocomposite Structural Ceramics with AdvancedProperties”, Proc. 4th Japan International SAMPE. (1995)]. It is in thisfield where the development of a simple, efficacious and cheap method toprepare metallic nanoparticles is of considerable interest, from theindustrial perspective, since it would allow the manufacture of newdevices based in the properties of the nanomaterials at very competitiveprices.

DESCRIPTION OF THE INVENTION

The objective of the present invention is to overcome most of theobstacles present in the current art by implementing a simple, economic,and viable procedure to prepare nanoparticles at the industrial scale byhomogenously dispersing a metallic compound in contact with a support,in which the support is at least a clay with a silicate network selectedfrom the pseudolaminar phyllosilicate clays. According to the presentinvention, the clay may be a sepiolite clay, including the naturalmineral sepiolites and the treated sepiolites such as rheologic gradesepiolite (as those marketed by TOLSA S.A, Madrid, Spain under thePANGEL brand and obtained from natural sepiolite by specialmicronization processes that substantially avoid fiber breakage anddescribed, for instance, in the patent applications EP-A-0170299 andEP-A-0454222), mineral or treated atapulgite, such as rheologic gradeatapulgite (like the one found in the ATTAGEL product range manufacturedand commercialized by Engelhard Corporation in the United States, andthe MIN-U-GEL product range offered by the Floridin Company, or thoseobtained by treating atapulgite with the process described in patentEP-A-0170299). Conveniently, the support is a powder which particle sizeis smaller than 44 μm and preferably smaller than 5 μm.

Sepiolite and atapulgite or palygorskite belong to the pseudolaminarphyllosilicate clays, also known as palygorskite-sepiolite group, whichstructure determines a microfibrous or acicular morphology.

Hence, sepiolite is a hydrated magnesic silicate, although there arealso aluminic sepiolites (in which 19% of the octahedral positions areoccupied by aluminum ions), ferric sepiolites (called xylotile),nickelferrous sepiolites (falcondoite) and sodic sepiolites(loughlinite). Palygorskite, or atapulgite, is a hydrated aluminummagnesium silicate with a structure similar to that of sepiolite.According to Brauner and Preisinger, sepiolite is structurally formed bytalcum type strands composed by two layers of silicon tetrahedronsjoined by oxygen atoms to a central layer of magnesium octahedrons.These talcum-type strands are arranged in such a manner that the silicontetrahedral layer is continuous, but the silicon tetrahedrons areinverted at intervals of six units. This structure determines theacicular morphology of the sepiolite particles, elongated along the axisc, and the presence of channels, called zeolitic channels, oriented inthe direction of the c axis of the acicular particles and measuring 3.7Å×10.6 Å, where water and other liquids can penetrate. As a result ofthis structure, sepiolite has a very high specific surface area that isdue not only to the high external surface, but also to the internalsurface originated by the zeolitic channels. The theoretical totalspecific surface of sepiolite, calculated in base to structural models,is of 900 m²/g., of which 400 m²/g belong to the external area and 500m²/g to the internal area. However, not all of the sepiolite surface isequally accessible to all molecules. The accessible surface of sepiolitedepends of the adsorbate used, of its size and of its polarity, whichdetermines the accessibility of the adsorbate molecule to the clay'smicropores and the zeolitic channels. The accessible BET surface to N₂is more than 300 m²/g, one of the highest surfaces found in a naturalcompound.

Atapulgite has a similar structure, although in this case the inversionof the silicon tetrahedrons occurs every other four tetrahedrons,instead of every other six as in the case of sepiolite. As a result, theatapulgite's zeolitic channels have a section of 3.7 Å×6.4 Å, that is,smaller than those of the sepiolite's channels, and therefore,atapulgite's specific surface, at approximately 150 m²/g, although high,is also smaller than that of sepiolite's.

The microfibrous particles of sepiolite and atapulgite are, in theirnatural state, arranged in clusters that form great bundles of acicularparticles randomly arrayed in a structure analogous to that of ahaystack. The structure thus formed is very porous and has a high volumeof mesopores and macropores. By using special milling and micronizationtechniques, such as those described in patent EP-A-0170299, it ispossible to de-agglomerate these microfiber beams into individualmicrofibrous particles while maintaining the high “aspect ratio”, thatis the length/diameter ratio. These procedures allow the adsorbedmolecules easier access to the external surface, therefore increasingthe surface accessible for adsorbtion. Treating sepiolite and atapulgitethermally in order to eliminate the water adsorbed on the surface, andparticularly, the water linked by hydrogen bridges to the watermolecules of crystallization that complete the coordination of themagnesium atoms—in the case of the sepiolite—or the magnesium andaluminum atoms—in the atapulgite's case-located at the edge of thestructure, both in the internal zeolitic channels as in the openchannels located at the edge of the structure, serves to also increasethe adsorption capacity of these clays.

Obtaining metallic nanoparticles over the clay's surface may be done byusing any of the paligorskite-sepiolite clays, for instance, sepiolite,atapulgite and their combinations, and sepiolite and/or atapulgitemineral provided they are present in a concentration in which the sumexceeds 50%, and preferably if it is 85%, since the contamination byother minerals such as calcite, dolomite, feldspar, mica, quartz orsmectite, besides diluting the clay over which the nanoparticles canform, can also affect the final properties of the product, as well asthe development of the process itself, both during the precipitation ofsalts, hydroxides or oxides, and during the thermal treatment appliedfor the reduction of the metal.

Also, the metallic compound is, at the least, a precursor selected fromthe salts, hydroxides and oxides of the metallic elements, and themetallic element is selected from metallic elements susceptible toreduction at temperatures that are below the temperature at which theclay silicate network collapses. Some adequate metallic elements are Fe,Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, W, Re, Os, Ir, Pt, Au and their alloysor combinations thereof. These metallic elements are present inprecursors such as hydrosoluble salts (chlorides, nitrates andsulphates).

The procedure described in the present invention also includes adeposition stage in which the precursor is deposited over the support,and when said precursor has been selected from salts and hydroxides, theprocedure also entails a thermal decomposition stage in controlledatmosphere in which the precursor is decomposed into the oxide of themetallic element of choice. Later, a reduction stage is carried out inwhich the oxide of the metallic element is subject to a completereduction process under controlled partial oxygen pressure (p0₂) andtemperature conditions to finally obtain metallic nanoparticlesdeposited over the support.

The procedure is done at temperatures below that at which the silicatenetwork of the clay would be destroyed, and preferably, at temperaturesbelow 850° C., since at higher temperatures the sepiolite and theatapulgite suffer profound structural transformations that lead to thedestruction of the silicate network as well as the potential appearanceof other stages, such as clinosteatite, in the case of the vitreousphase of sepiolite.

One of the embodiments of the deposition stage entails dissolving theprecursor in water to obtain a dilution of the precursor, disperse thesupport in said precursor dilution to obtain a precursor/supportdispersion, and dry the precursor/support dispersion to obtain dryprecursor/support particles. To enhance the degree of dispersion of theclay the preferred method is to apply mechanical stirring with highpowered shear blades.

Preferably, and depending on the desired results, the precursor dilutionis adjusted to a precursor concentration of 5 to 15%, the supportdiluted in water or in precursor dilution is adjusted to a 5 to 15%concentration. In addition, and conveniently, the support/precursordispersion may be adjusted to a metallic element/support ratio in arange from 0.1:100 to 100:100 as a function of the desired density ofthe nanoparticles to be obtained on the surface of the support, and morepreferably within a 5:100 to 50:100 range by weight.

When the aim is to precipitate the precursor over the support by raisingthe pH of the support/precursor dispersion this is achieved a basebefore the drying stage. The precipitation of precursors must be done ina controlled manner in order for the particles to deposit homogeneouslyover the clay's surface.

Also, the precursor/support dispersion is filtered before proceeding tothe drying stage, and/or is separated by solid/liquid separationtechniques before the drying stage. Preferably, a filtration orliquid/solid separation is done to separate the clay the metallicprecursors deposited over the surface of the solution containing themetallic salt ion used, although a direct drying can be done toevaporate all the water present in the dispersion. In some cases whenthe separation is done by filtering to separate the clay the metallicprecursors deposited, it is recommended to wash the clay to eliminateany trace of the soluble salt. The next step is the thermal treatmentcarried out under controlled atmosphere conditions to effect thedecomposition of the metallic salts or hydroxides in the appropriateoxide, provided the precursor deposited over the clay is not said oxide;the next step is the reduction of the metal oxide in the appropriatemetal. The conditions for the reduction (temperature and partial oxygenpressure) will depend on the metallic element used.

Utilizing the techniques described above, it is possible to obtainmonodispersed metallic nanoparticles deposited over the support, alwaysin a size smaller than 30 nm, and very frequently it is possible toobtain controlled sizes between 10 nm and 5 nm, and thus form ananoparticulate “nanocomposite” material useful for a diversity ofapplications. Thus, when the metallic element has been selected from Cu,Ag, Au, Rh, Pd, Ir, Ni, Pt and combinations thereof, the nanoparticulate“nanocomposite” material is useful as such, or as a component of acatalyst, while when the metallic element selected is Ag, thenanoparticulate “nanocomposite” material is useful as a biocide or as acomponent of biocides. In the same manner, when the metallic elementselected is Cu, the nanoparticulate “nanocomposite” material is usefulas fungicide or as a component of fungicide products.

Also, when the metallic element has been selected from Cu, Ag, Au andcombinations thereof, the nanoparticulate “nanocomposite” material isuseful as component of optoelectronic materials, while when the metallicelement has been selected from Fe, Ni, Co and combinations thereof thenanoparticulate “nanocomposite” material is useful as component offerromagnetic fluids.

In this process the clay used may have any particle size, although whenthe nanoparticles are formed on the clay particle's surface, it isadvisable to use a clay product with the smallest possible particle sizein order for the surface of the particle accessible to nanoparticlesformation to be the maximum possible size. In this manner the clay canbe added as milled powder with a particle size smaller than 44 μm.Rheologic grade products such as rheologic grade sepiolite can also beused, as obtained by means of a wet micronization processes such asthose described in the patent application EP-A-0170299, with which freeacicular particles have been obtained while maintaining the high “aspectratio” of the particles, and where the de-agglomeration process has leftmore free surface accessible to deposition. In addition, the colloidalproperties of the clays treated according to this micronization processhave higher stability and disperse better in the metallic salt dilution,allowing for a more homogenous coating.

The information described above indicates that the procedure describedin the present invention is based on the deposition over the surface ofthese metallic salts, oxides or hydroxide clays, followed by a reductiontreatment intended to obtain the corresponding metals. The reductionprocess is done by means of a thermal treatment carried out undercontrolled atmosphere conditions. It can be noted that the size of thenanoparticles formed by this process is smaller than 30 nm, and normallya size of approximately 3 nm, with a homogeneous distribution over thesurface and not clustered. The nanoparticles are distributed linearlyand oriented along the longitudinal axis of the microfibrous particles.A possible explanation for the formation of the metallic nanoparticlesin this particular arrangement may be the transformations undergone bythe sepiolite and the atapulgite during thermal treatments. At 350° C.the sepiolite losses two of the four molecules of hydration water,producing the folding of the sepiolite's structure, and the collapse ofthe zeolitic channels in order for the cations located at the edge ofthe octahedral layer of the silicate to complete their coordination withthe oxygen molecules of the tetrahedral layer of the adjacent silicon.This change is reversible. The original structure can be recovered byrehydrating the sepiolite. At a temperature of 500° C., the sepiolitelosses the two remaining water molecules of crystallization, andalthough there are no additional structural changes, the folding of thestructure becomes at this point irreversible. The atapulgite suffers asimilar structural change during thermal treatment. It is believed thatduring the thermal treatment carried out to decompose the metal salts orhydroxides in their corresponding oxides, and the subsequent step ofreducing the metallic oxides particles, these particles are trappedduring the folding and collapsing of the open channels located at theedge of the structures, preventing the migration of the metallicnanoparticles and their coalescence and growth processes that would formparticles of greater size. As a consequence, it is possible to obtainedmonodispersed nanoparticles which size is smaller than 30 nm that can becontrolled as a function of the impregnation conditions for the metallicsalts, hydroxides and oxides, and as a function of the reductionconditions, to obtain a controlled particle size that can be maintainedbetween 10 nm and 5 nm, and even to obtain particle sizes smaller than 5nm. This mechanism explains the linear arrangement of the nanoparticlesalong the axis of the microfibrous particle, following the open channelslocated at the edge of the structure over the surface of the acicularparticle.

BRIEF DESCRIPTION OF THE DRAWINGS

This section contains the description of some examples of theembodiments of the invention that will be referred to the followingfigures:

FIG. 1 a is a microphotograph of copper nanoparticles supported oversepiolite and obtained according to the method described in Example 1;

FIG. 1 b is a less enlarged microphotograph of the nanoparticlessupported over sepiolite and also obtained according to the methoddescribed in Example 1;

FIG. 2 a is an X-ray diffractogram showing copper precipitated asgerhardite over the sepiolite support according to Example 1;

FIG. 2 b is an X-ray diffractogram of the final product according toExample 1;

FIG. 3 shows the absorbance spectrum expressed in Kulbeka-Munk unitsmeasured by diffuse reflectance of the final product obtained as perExample 1.

FIG. 4 a is a microphotograph of silver nanoparticles supported oversepiolite and obtained according to the method described in Example 2.

FIG. 4 b is a less enlarged microphotograph of the nanoparticlessupported over sepiolite and obtained according to the method describedin Example 2;

FIG. 5 a is an X-ray diffractogram showing silver oxide precipitatedover the sepiolite support according to Example 2;

FIG. 5 b is an X-ray diffractogram showing the final product accordingto Example 2; and

FIG. 6 shows the absorbance spectrum in Kulbeka-Munk units measured bydiffuse reflectance of the final product obtained according to Example2;

FIG. 7 a is a microphotograph of copper nanoparticles supported overatapulgite and obtained according to the method described in Example 3;

FIG. 7 b is a less enlarged microphotograph of the nanoparticlessupported over atapulgite and obtained according to the method describedin Example 3;

FIG. 8 a is an X-ray diffractogram showing copper precipitated as copperhydroxide sulphate over the atapulgite support according to Example 3.

FIG. 8 b is an X-ray diffractogram showing the final product accordingto Example 3.

EXAMPLES OF THE DIFFERENT EMBODIMENTS OF THE INVENTION Example 1

The procedure begins by preparing one liter of copper dissolution (28,51 g of copper nitrate per liter), for a theoretical impregnation levelof 5%. Then the dissolution is acidified to pH 2 to insure the coppersalt is dissolved.

A dispersion of micronized sepiolite in water is prepared in a mixer, inwhich the size of 99.9% of the particles is smaller than 44 μm and thesize of 95% is smaller than 5 μm. The concentration of the dispersion is10% solids (150 g of dry sepiolite base for 1,500 g of pregel), and thedispersion mixed during 5 minutes by means of a mechanical stirrer. Thispredispersion of clay, which pH is approximately 9.0 is then acidifiedto pH 2 and then the copper dilution is added and the mixture stirredfor an additional 5 minutes to ensure the contact between the dilutionand the sepiolite is complete. The sepiolite dispersion in the copperdilution has a sepiolite concentration of 6%.

Later, the copper precipitates as gerhardite (Cu, (NO₃)₂(OH)₆) (FIG. 2a), by adding 1M sodium hydroxide until reaching a final pH=5.3. Thesodium hydroxide dilution is added slowly while the mixture is beingmechanically stirred. Once the copper hydroxide has precipitated thedispersion is filtered in a vacuum, washed and oven dried at 150° C.During this process it was observed that the BET specific surface of thesepiolite was reduced from 439 to 121 m²/g.

The sepiolite with the precursor is then subject to a reduction processin a tubular section oven with a controlled atmosphere of 10% H₂/90% Ar.The oven is outfitted with a programming device to control thetemperature (±1° C.). The reduction cycle entails heating the mixture inincreasing intervals of 10° C./min until reaching a temperature of 500°C. that is maintained for 2 hours, followed by a free cooling periodinside the oven.

As a result of this process, small copper nanoparticles are obtainedover the sepiolite fibers support. The nanoparticles appearmonodispersed and arranged parallel to the direction of the sepiolitefibers as can be observed in the attached microphotographs (FIG. 1 a andFIG. 1 b).

Finally, the calcination process, that is carried out in a H₂atmosphere, further decreases the surface until it reaches a value of 87m²/g.

The X-ray diffractogram of the reduced sample shows that the material isindeed composed of sepiolite and metallic copper (FIG. 2 b).

Additional evidence of the metallic nature of the nanoparticles thusobtained, as well as their dispersion pattern, was obtained by analyzingthe diffuse reflectance spectra within the visible ultraviolet range.When a metal finely divided (of a size smaller than the wavelength) anddispersed interacts with the electromagnetic radiation, it presents awell defined frequency, a collective electron excitation phenomenonknown as surface plasmon [C. F. Bohren and D. R. Huffman, “Absorptionand Scattering of Light by Small Spheres”, Ed. John Wiley and Sons, NewYork, 1983, pages 325 on]. In the frequency at which this phenomenonoccurs, it is verified that the real part of the dielectric constant ofthe metal is equal to minus two times the dielectric constant of thematrix (_(E) (w)=−2_(Em)). Since this frequency, which can beexperimentally recognized by an absorption maximum, is specific to eachmetal, it has served us as identification of the metallic nature of thenanoparticles. In the case of copper, the experimental spectrum shows amaximum absorption at 2.2. eV (FIG. 3) which is consistent with thevalue that would correspond to copper nanoparticles in air [“Handbook ofOptical Constants of Solids”, Edited by E. D. Palik, Academic Press,1985, Orlando, USA].

Example 2

A silver dilution containing 35.45 g of silver nitrate per liter isprepared and then acidified to pH=2 using NO₃H. A sepiolitepredispersion with a 10% concentration of solids is then added to thesilver nitrate dilution. The sepiolite predispersion is prepared bydispersing the sepiolite during 5 minutes in a mechanical stirrer withhigh powered shear blades to insure a good dispersion of the clayparticles. The sepiolite used in this example is a PANGEL rheologicgrade sepiolite manufactured by TOLSA S.A. Once the sepiolitepredispersion has been added to the silver nitrate dilution for theAg/sepiolite relationship to be in a15/100 ratio, the mixture is stirredat high shear setting during 5 more minutes and it is then, while stillstirring, a 1M NaOH solution is added slowly until pH=12. Increasing thepH produces the precipitation of the silver precursor that is thendeposited homogeneously over the sepiolite's surface. Later, thedispersion is filtered in a vacuum and oven dried at 150° C.

During this process, the BET specific surface of the sepiolite isdecreased from 439 to 204 m²/g.

The sepiolite with the silver precursor (Ag₂O in this particularinstance) (FIG. 5 a) is subjected to a reduction process in a tubularsectioned oven similar to that described in the previous case and thetemperature is set at 400° C.

The particles obtained as a result of this process are elongatedsepiolite particles over which small silver nanoparticles have appearedfollowing the direction parallel to the long axis of the sepiolitefibers. An image of these particles can be seen in FIG. 4 a and FIG. 4b. In this case some particles of about 15 nm can be seen together withsmall nanoparticles of a few nm.

Once the silver oxide particles have been reduced, the final specificsurface of the powder has decreased to 112 m²/g.

The X-ray diffractogram of the reduced sample shows that the material isindeed composed of sepiolite and silver (FIG. 5 b).

In the same manner as in the prior example, the optic absorption ofsepiolite samples with silver was measured in Kulbeka-Munk units bymeans of diffuse reflectance in the visible range of ultraviolet light.In this case the plasmon was also visible, but at a higher frequency(3.4 eV) and showing an irregular aspect as it is the case for silvernanoparticles (FIG. 6).

Example 3

A copper sulfate solution containing 79.11 g of copper sulfate per literis first prepared and then acidified by adding SO₄H₂ until obtaining apH value of 2. Later a predispersion of atapulgite in which the solidsconcentration=10% is added. The atapulgite predispersion is prepared bydispersing the atapulgite during 5 minutes in a mechanical stirrer withhigh powered shear blades to insure a good dispersion of the clayparticles. The atapulgite used is an ATTAGEL 40 from the EngelhardCorporation that has been micronized in wet conditions according to theprocedure described in patent EP-A-0170299. Once the atapulgitepredispersion has been added to the copper sulfate dilution for theCu/atapulgite relationship to be in a 15/100 ratio, the mixture isstirred at high shear setting during 5 more minutes and, while stillstirring, a 1M NaOH solution is added slowly until pH=5.5. Increasingthe pH produces the precipitation of one phase of the copper hydroxidesulfate (FIG. 8 a) that then deposits homogeneously over theatapulgite's surface. Later, the dispersion is filtered in a vacuum andoven dried at 150° C.

The atapulgite with the precursor is subjected to a reduction process ina tubular sectioned oven under controlled atmosphere conditions of 10%H₂/90% Ar. The reduction cycle is a heating process in which temperatureis increased at 10° C./min until reaching a final temperature of 500° C.which is maintained during 2 hours and followed by a free cooling periodinside the oven.

The particles obtained as a result of this process, are coppernanoparticles supported by atapulgite fibers. In this case acicularnanoparticles appear in a parallel arrangement following the directionof the atapulgite fiber. Their size is approximately 30 nm long by a fewnm wide (FIG. 7 b). When observing them in depth, it can be seen thatthese nanoparticles are made by nanoparticles clusters of approximately3 nm.

The X-ray diffractogram of the reduced sample shows that the material isindeed composed of atapulgite and metallic copper (FIG. 8 b).

While the invention has been described and illustrated herein byreference to the specific embodiments, various specific materials,procedures and examples, it is understood that the invention is notrestricted to the particular materials, combinations of materials, andprocedures selected for that purpose. Indeed, various modifications ofthe invention in addition to those described herein will become apparentto those skilled in the art from the foregoing description and theaccompanying figures. Such modifications are intended to fall within thescope of the appended claims.

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 19. A compound materialthat is composed of metallic nanoparticles deposited on the surface of asupport, said metallic nanoparticles being obtained by a method, inwhich the support is at least one clay possessing a silicate network andselected from sepiolite and atapulgite, a metallic precursor is at leasta precursor selected from salts, hydroxides and oxides of metallicelements, the metallic element is selected from metallic elementssusceptible to reduction at temperatures below the destructiontemperature of the silicate network of the clay, and wherein the methodalso comprises, an acidification stage of the metallic precursor, adeposition stage in which the precursor deposits on the support, athermal decomposition stage, when the precursor is selected from saltsand hydroxides, that is carried out under controlled atmosphereconditions in which the precursor is subjected to a decompositiontreatment during which the precursor is transformed into an oxide of themetallic element, and a thermal reduction stage carried out undercontrolled conditions in which the oxide of the metallic element issubjected to a reduction process in order to obtain nanoparticles of themetallic element deposited on the support, the procedure is carried outat temperatures below the destruction temperature of the clay's silicatenetwork and results in the obtaining of metallic nanoparticles with aparticle size smaller than 30 nm.
 20. A compound material according toclaim 19 wherein the metallic nanoparticles are homogeneously dispersedover a support, and in which said support is at least one clay,possessing a silicate network, that has been selected from sepiolite andatapulgite, in which the metallic nanoparticles are particles from anelement selected from Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, W, Re, Os, Ir,Pt, Au and alloys thereof, and in which the size of said metallicnanoparticles is smaller than 30 nm.
 21. A catalyst comprising acompound material of metallic nanoparticles deposited on the surface ofa support, being said compound obtained by a method, in which thesupport is at least one clay possessing a silicate network and selectedfrom sepiolite and atapulgite, the metallic precursor is at least aprecursor selected from salts, hydroxides and oxides of metallicelements, the metallic element is selected from metallic elementssusceptible to reduction at temperatures below the destructiontemperature of the silicate network of the clay, and wherein the methodalso comprises, an acidification stage of the metallic precursor, adeposition stage in which the precursor deposits on the support, athermal decomposition stage, when the precursor is selected from saltsand hydroxides, that is carried out under controlled atmosphereconditions in which the precursor is subjected to a decompositiontreatment during which the precursor is transformed into an oxide of themetallic element, and a thermal reduction stage carried out undercontrolled conditions in which the oxide of the metallic element issubjected to a reduction process in order to obtain nanoparticles of themetallic element deposited on the support, the procedure is carried outat temperatures below the destruction temperature of the clay's silicatenetwork and results in the obtaining of metallic nanoparticles with aparticle size smaller than 30 nm.
 22. A catalyst according to claim 21wherein the metallic element is selected from Cu, Ag, Au, Rh, Pd, Ir,Ni, Pt, and combinations thereof.
 23. A biocide material comprising acompound of metallic nanoparticles deposited on the surface of asupport, being said metallic nanoparticles obtained by a method in whichthe support is at least one clay possessing a silicate network andselected from sepiolite and atapulgite, the metallic precursor is atleast a precursor selected from salts, hydroxides and oxides of metallicelements, the metallic element is selected from metallic elementssusceptible to reduction at temperatures below the destructiontemperature of the silicate network of the clay, and wherein the methodalso comprises, an acidification stage of the metallic precursor, adeposition stage in which the precursor deposits on the support, athermal decomposition stage, when the precursor is selected from saltsand hydroxides, that is carried out under controlled atmosphereconditions in which the precursor is subjected to a decompositiontreatment during which the precursor is transformed into an oxide of themetallic element, and a thermal reduction stage carried out undercontrolled conditions in which the oxide of the metallic element issubjected to a reduction process in order to obtain nanoparticles of themetallic element deposited on the support, the method is carried out attemperatures below the destruction temperature of the clay's silicatenetwork and results in the obtaining of metallic nanoparticles with aparticle size smaller than 30 nm, and wherein the metallic element isAg.
 24. A fungicide material comprising a compound material of metallicnanoparticles deposited on the surface of a support, being said metallicnanoparticles obtained by a method in which the support is at least oneclay possessing a silicate network and selected from sepiolite andatapulgite, the metallic precursor is at least a precursor selected fromsalts, hydroxides and oxides of metallic elements, the metallic elementis selected from metallic elements susceptible to reduction attemperatures below the destruction temperature of the silicate networkof the clay, and wherein the method also comprises, an acidificationstage of the metallic precursor, a deposition stage in which theprecursor deposits on the support, a thermal decomposition stage, whenthe precursor is selected from salts and hydroxides, that is carried outunder controlled atmosphere conditions in which the precursor issubjected to a decomposition treatment during which the precursor istransformed into an oxide of the metallic element, and a thermalreduction stage carried out under controlled conditions in which theoxide of the metallic element is subjected to a reduction process inorder to obtain nanoparticles of the metallic element deposited on thesupport, the method is carried out at temperatures below the destructiontemperature of the clay's silicate network and results in the obtainingof metallic nanoparticles with a particle size smaller than 30 nm, andwherein the metallic element is Cu.
 25. An optoelectronic materialcomprising a compound material of metallic nanoparticles deposited onthe surface of a support, being said metallic nanoparticles obtained bya method, in which the support is at least one clay possessing asilicate network and selected from sepiolite and atapulgite, themetallic precursor is at least a precursor selected from salts,hydroxides and oxides of metallic elements, the metallic element isselected from metallic elements susceptible to reduction at temperaturesbelow the destruction temperature of the silicate network of the clay,and wherein the method also comprises, an acidification stage of themetallic precursor, a deposition stage in which the precursor depositson the support, a thermal decomposition stage, when the precursor isselected from salts and hydroxides, that is carried out under controlledatmosphere conditions in which the precursor is subjected to adecomposition treatment during which the precursor is transformed intoan oxide of the metallic element, and a thermal reduction stage carriedout under controlled conditions in which the oxide of the metallicelement is subjected to a reduction process in order to obtainnanoparticles of the metallic element deposited on the support, themethod is carried out at temperatures below the destruction temperatureof the clay's silicate network and results in the obtaining of metallicnanoparticles with a particle size smaller than 30 nm, and wherein themetallic element is selected from Cu, Ag, Au and combinations thereof.26. A ferromagnetic fluid comprising a compound material of metallicnanoparticles deposited on the surface of a support, being said metallicnanoparticles obtained by a method in which the support is at least oneclay possessing a silicate network and selected from sepiolite andatapulgite, the metallic precursor is at least a precursor selected fromsalts, hydroxides and oxides of metallic elements, the metallic elementis selected from metallic elements susceptible to reduction attemperatures below the destruction temperature of the silicate networkof the clay, and wherein the method also comprises, an acidificationstage of the metallic precursor, a deposition stage in which theprecursor deposits on the support, a thermal decomposition stage, whenthe precursor is selected from salts and hydroxides, that is carried outunder controlled atmosphere conditions in which the precursor issubjected to a decomposition treatment during which the precursor istransformed into an oxide of the metallic element, and a thermalreduction stage carried out under controlled conditions in which theoxide of the metallic element is subjected to a reduction process inorder to obtain nanoparticles of the metallic element deposited on thesupport, the method is carried out at temperatures below the destructiontemperature of the clay's silicate network and results in the obtainingof metallic nanoparticles with a particle size smaller than 30 nm, andwherein the metallic element has been selected from Fe, Ni, Co andcombinations thereof.